Trans RINA, Vol 153, Part A4, Intl J Maritime Eng, Oct-Dec 2011
stiffening up of the vibration affected structures. The first sign of potential propeller-cavitation-induced-vibration is manifested in the aft stern area of the vessel. Generally, vibration in the aft stern area of the vessel is due to frequencies coming from the wakefield, propeller or engine, exciting local
structures ranging from the
accommodation deck to aft mooring deck and lifeboat support structures. Once the excitation frequencies coincide with any of the structures’ natural frequencies, a resonance such as a high amplitude vibration can occur leading to possible fatigue and failure.
Figure 2a: Cavity volume variation through one revolution of the propeller.
Assuming the source of the problem is suspected to be hydrodynamic, then additional tests and observations need to be conducted, such as underwater pressure measurements in the area of the propeller, to establish potential blade overloading and pressure pattern analysis. In addition, underwater
stroboscopic cameras can be installed to observe the propeller interaction with the
cameras or underwater wakefield.
The
measurements need ideally to be accompanied by either model
tank tests or additional Computational fins, vortex Fluid
Dynamics (CFD) simulation of the wakefield for further comprehension of the problem. If a propeller cavitation problem is identified, then potential wake correction devices, such as flow fairings, stern
generators, etc., can intervene into the wakefield pattern and correct or improve the flow in favour of the reduction of the propeller cavitation intensity albeit at the expense of some increase in the ship drag resistance. Changing the propeller from a four-bladed propeller to a propeller with larger blade areas or a propeller with five or six blades could reduce or eliminate the cavitation as the blade area load or blade area pressure would be reduced to lower levels. As a practical rule of thumb, a good safe loading for the propeller blades is about 6.4 lb/in2 or 450 g/cm2
3.1 STRUCTURAL RESPONSE
Once a problem such as general vibration in the aft part of the vessel is reported, then overall Root Mean Square (RMS) vibration levels are conducted to establish objective measurements against set criteria. Figure 3. shows the overall RMS vibration levels as originally measured on board an LNG carrier vessel with a four- bladed fixed pitch propeller under different
service
Figure 2b: Frequency content of the cavitation signal including harmonic, periodic and impact information.
3 CASE STUDY
ABS have witnessed a number of cases in the recent years, where it is too late or too costly for the vessel operator to request to have the propeller replaced due to cavitation and cavitation-induced vibration. Instead, the operator has to live with high vibration levels potentially until the propeller becomes eroded and commences
©2011: The Royal Institution of Naval Architects
operating conditions. From the measurements, it can be seen that the problematic areas are the helideck, the aft mooring deck of the vessel and the steering gear room, where the levels exceed the overall RMS levels found in ISO 6954.
Concentrating on the areas of highest vibration level, a frequency analysis of the vibration signal at Maximum Continuous Rating, (MCR) speed is further carried out in the steering gear room and the helideck. The results shown in Figures 4a and 4b show the existence of a strong amplitude
propeller blade passing together with its higher multiples including up to the
frequency
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